Electrically conductive lines and connections form many common components of integrated circuits. Dynamic random access memory (DRAM) circuitry, for example, incorporates multiple parallel conductive lines to form word-lines and bit-lines, which must be connected to various components. In order to increase capacity and accommodate smaller devices, there is constant pressure to increase the density of components on these and other circuits. The continual reduction in feature size places greater demands on the techniques used to form the features.
Photolithography is a commonly used technique for patterning integrated circuit features, such as conductive lines and vias that may be filled with a conductive material to form a connection. One example of a photolithographic method for patterning integrated circuit features includes depositing a photoresist material over a material to be patterned, covering portions of the photoresist material with a mask, exposing the uncovered photoresist material to light, and etching away either the exposed portion, in the case of a positive resist, or the unexposed portion of the photoresist material, in the case of a negative resist. The remaining photoresist material is used as an etch mask for an etching process. In the etching process, portions of the of the material to be etched that are not covered by the photoresist material are removed by, for example, wet or dry chemical etch. After the etch, the remaining photoresist material is dissolved.
There are, however, limitations on how close features can be patterned using known photolithographic techniques. The size of features on an integrated circuit are conventionally described by their “pitch,” which is the distance between an identical point on two neighboring features. Due to an inherent resolution limit, which is a function of a numerical aperture of the mask and the wavelength of the light used, there is a minimum pitch below which features cannot be reliably formed using conventional photolithographic techniques.
Thus, there exists a need for a method and apparatus to pattern closely spaced features having a smaller pitch than would be possible using a mask having a given resolution limit in a conventional photolithographic technique.